Electronic apparatus having audio output units

ABSTRACT

Two control voltages are generated, one increasing and another decreasing, in response to the rise of the voltage of a condenser of a common time-constant circuit. Based on the two control voltages, a first and a second currents are formed, from which four different combinations of currents are established. By feeding one of four combinations to each of multiple differential output amplifier circuits each having a feedback resistor, the multiple output amplifier circuits can be individually started up or shut down without generating popcorn noise, using only one single time constant circuit.

FIELD OF THE INVENTION

[0001] This invention relates to an electronic apparatus having amultiplicity of audio output units for outputting sounds with reducedpopcorn noise during a startup and a shutdown of the audio output unit.

BACKGROUND OF THE INVENTION

[0002] It is often the case that electronic apparatuses having audiooutput units generate unpleasant audible popcorn noise due to a suddenchange of a bias voltage during a startup and a shutdown of the audiooutput unit. In order to prevent or suppress generation of such popcornnoise, the audio output unit is designed to have a sufficiently largetime constant to allow the bias voltage rise and fall slowly during itsstartup and shutdown, respectively.

[0003] When the apparatus has a multiplicity of output amplifiercircuits for a CD player and a MD player for example which are startedup or shut down simultaneously, their bias voltages can be raised anddropped simultaneously using a common time constant circuit.

[0004] However, if the electronic apparatus has audio output unitsadapted to individually and independently start up or shut down amultiplicity of output amplifier circuits, the output amplifier circuitscannot be controlled by a single time constant circuit. In this case,each output amplifier circuit requires its own time constant circuit.

[0005] Take a cellular phone for example. It has four output amplifiercircuits for an earphone, a back loudspeaker, a receiver, and anexternal output. Since cellular phones are powered by a battery, theirpower consumption is preferably reduced as much as possible to prolongthe life of the battery. In a cellular phone, therefore, the outputamplifier circuits are designed to operate in such a way that only oneoutput amplifier circuit currently in use is powered but the rest of theoutput amplifier circuits are shut down. Thus, a multiplicity of timeconstant circuits are provided one for each output amplifier circuit toallow the individual output amplifier circuits to be started up and shutdown without generating popcorn noise.

[0006] These time constant circuits respectively require a condenser anda charge and discharge circuit. These output amplifier circuits occupy alarge area on an IC chip, since each of the output amplifier circuitsincludes a condenser. This hinders miniaturization of the IC chip. Ifthese condensers are externally provided, the IC chip requiresadditional connection pins. In addition, a space is needed forinstalling the condensers.

SUMMARY OF THE INVENTION

[0007] It is, therefore, an object of the invention is to provide anelectronic apparatus having a multiplicity of audio output units capableof independently starting up and shutting down a multiplicity of outputamplifier circuits, without generating popcorn noise, using a singletime constant circuit.

[0008] In accordance with one aspect of the invention, there is providedan electronic apparatus having audio output units, comprising:

[0009] a multiplicity of output amplifier circuits each having

[0010] a differential amplifier having a first input terminal forreceiving an audio input signal, a second input terminal for receiving areference voltage, and an output terminal for outputting the outputsignal generated by said amplifier circuit, and

[0011] a feedback resistor connected between said first input terminaland said output terminal, said first input terminal capable of receivingan externally supplied DC current;

[0012] a common time constant circuit having a condenser and a chargeand discharge circuit connected to said condenser, said time-constantcircuit receiving a direction instruction signal instructing charging ordischarging of said condenser, and charging said condenser to a firstpredetermined voltage (the voltage across said condenser hereinafterreferred to as charging voltage) with a predetermined time constant uponreceipt of a direction instruction signal instructing charging of saidcondenser, but discharging said condenser to a second charging voltagewith said predetermined time constant upon receipt of a directioninstruction signal instructing discharging of said condenser, said timeconstant circuit thereby outputting said charging voltage of saidcondenser;

[0013] a common current instruction circuit receiving said chargingvoltage, said common current instruction circuit outputting a firstvoltage that increases (decreases) when said charging voltage increases(decreases) (said first voltage referred to as first direction voltage)and a second voltage that decreases (increases) when said chargingvoltage increases (decreases) (said second voltage referred to as seconddirection voltage); and

[0014] a multiplicity of current control circuits, each connected to acorresponding one of said output amplifier circuits and each

[0015] receiving said first direction voltage, second direction voltage,a first direction current selection signal, and a second directioncurrent selection signal, and

[0016] selectively outputting either one of a first direction current tosaid corresponding output amplifier circuit in accord with said firstdirection voltage, a second direction current in accord with said seconddirectional voltage, a sum of said first and second direction currents,or zero current according to the combination of said first and seconddirection current selection signals.

[0017] In the inventive electronic apparatus having audio output units,if the current state of the output amplifier circuit is to be changed,either one of the first and second direction selection switches isselected based on the current state of the output amplifier circuit(i.e. state of the output amplifier circuit being shut down or inoperation) and the current state of the charged condenser. Under thiscondition, the state of the time constant circuit is slowly inverted.Thus, any one or more of the multiple output amplifier circuits can beindividually started up and shut down without generating popcorn noise,using a single time-constant circuit.

[0018] In the invention, a transistorized differential amplifier circuitis used as a common current instruction circuit such that thedifferential amplifier circuit is controlled by the charging voltage andthe reference voltage. This enables, in an easy and accurate way,establishing the first direction voltage that increases with thecharging voltage, and a second direction voltage that decreases with thecharging voltage (implying that the second direction voltage decreasesas the charging voltage increases).

[0019] Further, a first series circuitry of a first direction currentcontrol element and a first direction selection switch and a secondseries circuitry of a second direction current control element and asecond direction selection switch are connected together in parallelsuch that the switching of the first and the second direction selectionswitches can be controlled. Thus, the currents determining the biases ofthe respective output amplifier circuits can be easily controlledindividually and independently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows an overall structure of audio output units of anelectronic apparatus according to the invention.

[0021]FIG. 2 shows an output amplifier circuit according to theinvention.

[0022] FIGS. 3(a)-(b) are graphical representations of a current i andan output voltage Vo1 during a startup and after a shutdown of theoutput amplifier circuit.

[0023]FIG. 4 is a circuit diagram of a current control circuit accordingto the invention.

[0024]FIG. 5 illustrates state transitions of an audio output unitaccording to the invention.

[0025]FIG. 6 shows control conditions for state transitions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] The invention will now be described in detail by way of examplewith reference to the accompanying drawings. In what follows the term“audio” means “sound” and “acoustic”.

[0027]FIG. 1 shows an overall structure of audio output units of anelectronic apparatus embodying the invention. FIG. 2 shows a structureof an output amplifier circuit shown in FIG. 1. FIG. 3 are graphsillustrating operations of the output amplifier circuit during a startupand a shutdown of the circuit. FIG. 4 shows a structure of a currentcontrol circuit shown in FIG. 1.

[0028] The audio output unit of FIG. 1 has a multiplicity of outputamplifier circuits 10-1-10-n for respectively amplifying audio inputsignals sig-1-sig-n received to generate output signals Vol-Von. Each ofthese output amplifier circuits 10-1-10-n is operable at a predeterminedbias voltage. In what follows the term “voltage” refers to the potentialwith respect to the ground, unless otherwise stated.

[0029] The present invention enables a multiplicity of output amplifiercircuits 10-1-10-n to be started up and shut down individually andindependently without generating a popcorn noise. To do so, theinvention provides a common time constant circuit 20, a common limitercircuit 25, a common current instruction circuit 30, and current controlcircuits 40-1-40 n one for each of the output amplifier circuits10-1-10-n, all connected for a cooperative operation.

[0030] These components may be formed on one IC chip, or on multiple ICchips with their components grouped on the respective IC chips.

[0031]FIG. 2 shows the structure of the output amplifier circuit 10-1.Other output amplifier circuits 10-2-10-n also have the same structure.As shown in FIG. 2, an operational amplifier OP of the circuit 10-1 issupplied at the non-inverting input terminal (+) thereof with areference voltage Vcom, and supplied at the inverting input terminal (−)thereof with an audio input signal sig-1 via a switch 11 and an inputresistor 12. The switch 11 is turned on while amplifying and outputtingthe audio input signal sig-1. A feedback resistor 13 of resistance Rf isconnected between the inverting input terminal (−) and the outputterminal of the operational amplifier OP.

[0032] The inverting input terminal (−) of the operational amplifier OPis supplied with a current i from a current control circuit 40-1. Thecurrent i flows into the output terminal of the operational amplifier OPvia the feedback resistor 13.

[0033] The operational amplifier OP is adapted to equalize the voltageapplied to the inverting input terminal (−) with the reference voltageVcom applied to non-inverting input terminal (+). Therefore, the outputvoltage (or bias voltage) Vo1 of the operational amplifier OP equals thereference voltage Vcom minus the voltage drop across the feedbackresistor 13, i.e. Vo1=Vcom−i×Rf. In actuality, the output voltage Vo1 isthe amplified audio input signal sig-1 superimposed with a DC voltage.However, the AC component of the output signal, i.e. audio signal, willnot be mentioned in the description below.

[0034] Referring to FIG. 3, operations of the output amplifier circuit10-1 at the time of a startup and a shut down will now be described.FIG. 3(a) shows the change of current i, and FIG. 3(b) shows the changeof the output voltage Vo1.

[0035] The output amplifier circuit 10-1 is biased by a predeterminedoutput voltage (which is the reference voltage Vcom in the example shownherein) when it is performing amplification, up to time t1 in FIG. 3(a).Therefore, the current i is zero up to time t1.

[0036] As the output amplifier circuit 10-1 is stopped, the current ibegins to gradually increase from zero at time t1, as shown in FIG.3(a), so as to gradually decrease the output voltage Vo1. The outputvoltage Vo1 becomes zero when the product i×Rf of the current i and thefeedback resistance Rf equals the reference voltage Vcom at time t2. Atthis moment, the current i equals the predetermined level (Vcom/Rf).

[0037] The power supply voltage Vcc, supplied to the output amplifiercircuit 10-1 and the current control circuit 40-1 which is the currentsource supplying the current i, is shut down while the output voltageVo1 is zero. As a consequence, no popcorn noise will be generatedaccompanying the shutdown of the power supply voltage Vcc.

[0038] To start up the output amplifier circuit 10-1 at time t3 say, thesource voltage Vcc is turned on, to thereby supply power to the outputamplifier circuit 10-1 and the current control circuit 40-1. This causesthe current i (=Vcom/Rf) to flow. Since the output voltage Vo1 is zeroat the time the source voltage Vcc is supplied, no popcorn noise will begenerated then, either.

[0039] The current i is then gradually decreased from the predeterminedlevel (=Vcom/Rf), starting at time t3, as shown in FIGS. 3(a)-(b). Thiscauses the output voltage Vo1 to gradually increase. The output voltageVo1 reaches the reference voltage Vcom when the current i becomes zeroat time t4. This completes the startup of the output amplifier circuit10-1, rendering the output circuit 10-1 operable under normal condition.Under this condition, the switch 11 is turned on to amplify the inputaudio signal sig-1 which is then outputted. During the amplifyingoperation, the current i is maintained at zero.

[0040] In this way, the current i is gradually increased when the outputamplifier circuit 10-1 is stopped, and the current i is graduallydecreased when the output amplifier circuit 10-1 is started up. It isseen that the power to the output amplifier circuit 10-1 is turned onand off while the output voltage Vo1 remains zero. This preventsgeneration of popcorn noise during a startup and a shutdown.

[0041] Referring again to FIG. 1, the time constant circuit 20 includesa switch 21, a constant current sources 22 supplying a constant currentI, and a charge and discharge condenser Cp connected in series betweenthe power supply voltage Vcc and the ground. Connected in parallel withthe condenser Cp is a p-type MOS transistor (referred to as p-typetransistor) Q23 connected in series with a constant current source 24supplying a constant current 2I. The p-type transistor Q23 is turned onand off by a direction control signal dir. The switch 21 is alwaysturned on while the audio output unit is in operation.

[0042] In the time constant circuit 20, when the direction controlsignal dir is at a high level HIGH and the p-type transistor Q23 isturned off, the condenser Cp is charged by the constant current I.Therefore, its charging voltage Vcp increases linearly. Conversely, whenthe direction control signal dir is at a low level LOW and the p-typetransistor Q23 is turned on, the condenser Cp discharges a constantcurrent I (=2I−I). Therefore, its charging voltage Vcp decreaseslinearly. The condenser Cp may be provided as an external component, notincluded in the IC chip.

[0043] The limiter circuit 25 limits the upper and the lower limits ofthe charging voltage Vcp to a predetermined upper level Vh and apredetermined lower level Vl, respectively. The upper and lower limitsVh and Vl, respectively, are set in accordance with the dynamic range ofthe current instruction circuit 30 in the next stage. This limitercircuit 25 may be included in the time constant circuit 20.

[0044] In the example shown herein, the current instruction circuit 30is constructed in the form of a current difference circuit. The currentinstruction circuit 30 has an n-type MOS transistor (hereinafterreferred to as n-type transistor) Q31 adapted to receive at the gatethereof the charging voltage Vcp as a control signal, and an n-typetransistor Q32 adapted to receive at the gate thereof a predeterminedreference voltage Vb. The sources of the n-type transistors Q31 and Q32are connected together and further connected to the constant currentsource 32 adapted to flow a predetermined current.

[0045] Thus, these n-type transistors Q31 and Q32 forms paireddifferential transistors, with the reference voltage Vb, coupled to thegate of the transistor Q32, is set to an intermediate voltage betweenthe upper limit Vh and the lower limit V1 of charging voltage Vcp.

[0046] The drain of the n-type transistor Q31 is connected to the sourcevoltage Vcc via a p-type transistor Q33 whose gate and drain are coupledtogether. The drain of the n-type transistor Q32 is connected to thesource voltage Vcc via a p-type transistor Q34 whose gate and drain arecoupled together. It is noted that each of these p-type transistors Q33and Q34 can be substituted for by a respective load resistor.

[0047] The current instruction circuit 30 outputs, from the drain of then-type transistor Q31, a voltage Vcf that decreases with the chargingvoltage Vcp (the decreasing voltage referred to as the second directionvoltage or positive current instruction signal). On the other hand, avoltage Vcr that increases with the charging voltage Vcp (said voltagereferred to as the first direction voltage or reverse currentinstruction signal) is outputted from the drain of the n-type transistorQ32.

[0048] The current control circuits 40-1-40-n are provided one for eachof the output amplifier circuits 10-1-10-n. The current control circuits40-1-40-n are supplied with a common positive current instruction signalVcf and a common reverse current instruction signal Vcr. The currentcontrol circuits 40-1-40-n are also individually supplied withrespective positive current selection signals ct-1 f-ct-nf and reversecurrent selection signals ct1 r-ct-nr.

[0049] Referring to FIG. 4, there is shown a current control circuit40-1, other current control circuits 40-2-40-n have a structure similarto that of the circuit 40-1.

[0050] As shown in FIG. 4, the positive current instruction signal Vcfis applied to the gate of the p-type transistor Q41 serving as apositive current control element, while a positive current selectionsignal ct-1 f is applied to the gate of a p-type transistor Q43 servingas a selection switch for selecting the direction of positive current(the switch referred to as positive-direction selection switch). Thesetransistors Q41 and Q43 are connected in series.

[0051] Conductivity of the p-type transistor Q41 is controlled by thepositive current instruction signal Vcf. Hence, when the p-typetransistor Q43 is turned on, a current flows through this first seriescircuit in accord with the instruction signal Vcf.

[0052] The reverse current instruction signal Vcr is applied to the gateof a p-type transistor Q42 serving as an element for controlling reversecurrent (the element referred to as reverse current controllingelement), while the reverse current selection signal ct-1 r is appliedto the gate of a p-type transistor Q44 serving as a switch for selectingthe reverse direction of current (the switch referred to asreverse-direction selection switch). These transistors Q42 and Q44 areconnected in series. The conductivity of the p-type transistor Q42 iscontrolled by the reverse current instruction signal Vcr. Thus, when thep-type transistor Q44 is turned on, a current flows through this secondseries circuit in accord with the instruction signal Vcr.

[0053] Since the first and the second series circuits are connected inparallel with each other, a combined current i is supplied to the outputamplifier circuit 10-1 in the next stage.

[0054] Thus, a current i indicative of the magnitude of the positivecurrent instruction signal Vcf, magnitude of the reverse currentinstruction signal Vcr, and levels (LOW or HIGH) of the positive currentselection signal ct-1 f and reverse current selection signal ct-1 r isselectively outputted from the current control circuit 40-1. The currenti outputted selectively is associated with either:

[0055] the positive current instruction signal Vcf; the reverse currentinstruction signal Vcr; or the positive current instruction signal Vcfplus the reverse current instruction signal Vcr, or zero current.

[0056] Referring to FIG. 5, there is shown different types of statetransitions involved in the audio output unit. FIG. 6 shows conditionsgoverning the state transitions. Operations of the electronic apparatusof the invention will now be described below.

[0057] In this invention, the apparatus has a multiplicity of outputamplifier circuits 10-1-10-n associated with the respective audio outputunits. Each of these output amplifier circuits can be operated (inoperating state) or stopped (in inoperable state) arbitrarily, and canchange its state from one to the other independently and individually.Moreover, any of the output amplifier circuits 10-1-10-n will notgenerate a popcorn noise as they change their states, as will be seenbelow.

[0058] Each of the output amplifier circuits 10-1-10-n has four possiblestates A-D as shown in FIG. 5. These states A-D taken by the respectiveoutput amplifier circuits 10-1-10-n are determined by the conditions ofthe time constant circuit 20 and the current control circuit 40-1-40-n.

[0059] In state A, the charging voltage Vcp is high (voltage Vh), andthe output voltage Vo1 is zero (0). In state B, the charging voltage Vcpis low (voltage V1), and the output voltage Vo1 is zero (0). In state C,the charging voltage Vcp is high Vh, and the output voltage Vo1 is Vcom(reference voltage). In state D, the charging voltage Vcp is low V1, andthe output voltage Vo1 is Vcom. That is, in states A and B, the outputamplifier circuits 10-1-10-n are disabled, and in state C and D, theoutput amplifier circuits 10-1-10-n are in operation.

[0060] Bidirectional inter-state transitions are possible between stateA and state B, state A and state D, state B and state C, and state C andstate D.

[0061]FIG. 6 shows possible state transitions among the states A-D interms of the direction control signal dir and the positive currentselection signal ct-1 f-ct-nf, or reverse current selection signal ct-1r-ct-nr in combination. It should be understood that FIG. 6 showstransitions occurring in the exemplary output amplifier circuit 10-1.

[0062] State transitions are shown by arrows in FIG. 6. For example,symbol “A→B” represents a state transition from state A to state B.Symbols “LOW” and “HIGH” for the signal dir represent the levels of thesignal dir, rendering the p-type transistor Q23, controlling thedirection of the transition, turned on or off, respectively. Symbols“LOW” and “HIGH” for the positive direction current selection signalct-1 f represent the levels of the signal ct-1 f, rendering thepositive-direction selection switch Q43 turned on or off, respectively.Similarly, symbols “LOW” and “HIGH” for the reverse current selectionsignal ct-1 r represent the levels of the signal ct-1 r, rendering thereverse-direction selection switch Q44 turned on or off. It will beunderstood that the logical operations of the positive-directionselection switch Q43 and reverse selection switch Q44 are inverted,since they are p-type transistors.

[0063] A preferred state of the audio output unit can be obtained bychoosing an appropriate combination of the direction control signal dirand positive current selection signal ct-1 f-ct-nf or reverse currentselection signal ct-1 r-ct-nr that corresponds to a state transitionleading to the preferred state of the audio output unit, as shown inFIG. 6. In this way, arbitrary one or more than one of the outputamplifier circuits can be independently and individually started up andshut down without affecting other output amplifier circuits withoutgenerating popcorn noise by the use of a single time constant circuit.

[0064] As an example, operations of the output amplifier circuit 10-1will be described below. To begin with, a state transition from state Ato state D is considered. Before the transition, the output amplifiercircuit is in state A, where the charging voltage Vcp is high (Vh), thepositive current instruction signal Vcf low, and the positive selectionsignal ct-1 f low, so that the current i flowing through the outputamplifier circuit 10-1 has a predetermined magnitude. Consequently, theoutput voltage Vo1 is zero. That is, the output amplifier circuit 10-1is shut down.

[0065] Under this condition, the reverse selection signal ct-1 r ispulled up to HIGH while keeping the positive selection signal ct-1 fLOW, and the direction control signal dir is pulled down to LOW to turnon the p-type transistor Q23. This caused the charging voltage Vcp to begradually lowered, and causes the positive current instruction signalVcf to be gradually increased, thereby reducing the current i. As aconsequence, the output voltage Vo1 is gradually increased, until itreaches the reference voltage Vcom when the current i becomes zero.

[0066] Next, a transition from state A to state B will be considered.This transition takes place when the charging voltage Vcp is loweredfrom the high charging voltage Vh to the constant voltage Vl to hold theoutput amplifier circuit 10-1 shut down but allow other output amplifiercircuit to be started up or shut down.

[0067] In this case, while keeping both the positive selection signalct-1 f and the reverse selection signal ct-1 r LOW as they are in stateA prior to the transition, the direction control signal dir is pulleddown to LOW to turn on the p-type transistor Q23. This causes thecharging voltage Vcp to slowly decrease and causes the positive currentinstruction signal Vcf to rise slowly, thereby slowly lowering thereverse current instruction signal Vcr. As a result, the current i,which is the sum of the currents flowing through the two paths,invariably remains constant. Hence, the output voltage Vol invariablyremains at zero volt.

[0068] In this way, the output amplifier circuit 10-1 remains shut downwithout being affected by the change of the charging voltage Vcpintroduced to start up or shut down other output amplifier circuits.

[0069] Although details are omitted, it will be understood that atransition from state C (operating state) to state B (non-operatingstate) is possible, and so is a transition from state C to anotheroperating state D. These transitions among states A-D are carried outwhen relevant transition conditions listed in the table of FIG. 6 aresatisfied.

What we claim is:
 1. An electronic apparatus having audio output units,comprising: a multiplicity of output amplifier circuits each having adifferential amplifier having a first input terminal for receiving anaudio input signal, a second input terminal for receiving a referencevoltage, and an output terminal for outputting its output signal, and afeedback resistor connected between said first input terminal and saidoutput terminal, said first input terminal capable of receiving asupplied DC current; a common time-constant circuit having a condenserand a charge and discharge circuit connected to said condenser, saidtime-constant circuit receiving a direction instruction signalinstructing charging or discharging of said condenser, and charging saidcondenser to a first predetermined voltage with a predetermined timeconstant upon receipt of a direction instruction signal instructingcharging of said condenser, but discharging said condenser to a secondcharging voltage with said predetermined time constant upon receipt of adirection instruction signal instructing discharging of said condenser,said time-constant circuit thereby outputting the charging voltage ofsaid condenser; a common current instruction circuit receiving saidcharging voltage, said common current instruction circuit outputting afirst direction voltage that increases (decreases) as said chargingvoltage increases (decreases) and a second direction voltage thatdecreases (increases) as said charging voltage increases (decreases);and a multiplicity of current control circuits, each current controlcircuit connected to a corresponding one of said output amplifiercircuits and receiving said first direction voltage, second directionvoltage, a first direction current selection signal, and a seconddirection current selection signal, and selectively outputting, to saidcorresponding output amplifier circuit, either one of a first directioncurrent in accord with said first direction voltage, a second directioncurrent in accord with said second directional voltage, a sum of saidfirst and second direction currents, or zero current according to thecombination of said first and second direction current selectionsignals.
 2. The electronic apparatus according to claim 1, wherein whenany one of said output amplifier circuits is started up, power issupplied to said output amplifier circuit while the output voltage ofthe differential amplifier of said output amplifier circuit is zero, andwhen any one of said output amplifier circuits is shut down, the powerto said output amplifier circuit is cut down while the output voltage ofsaid differential amplifier is zero.
 3. The electronic apparatusaccording to claim 1, wherein when any one of said output amplifiercircuits is started up, power is supplied to said output amplifiercircuit and the current control circuit associated with said outputamplifier circuit while the output voltage of the differential amplifierof said output amplifier circuit is zero, and when any one of saidoutput amplifier circuits is shut down, the power to said outputamplifier circuit and current control circuit is cut down while theoutput voltage of the differential amplifier is zero.
 4. The electronicapparatus according to claim 1, wherein said time-constant circuit has avoltage limiter circuit for limiting the upper and lower limits of saidcharging voltage to a respective predetermined upper and lower voltage.5. The electronic apparatus according to claim 1, wherein said chargeand discharge circuit includes a first constant current source forsupplying a first constant current, a charge/discharge instructionswitch adapted to be turned on and off by said direction instructionsignal, and a second constant current source supplying a second constantcurrent which is double said first constant current, all connected inseries, said charge and discharge circuit outputting a charging voltageof said condenser from the node of said first and second constantcurrent sources.
 6. The electronic apparatus according to claim 1,wherein said current instruction circuit includes a differentialamplifier circuit consisting of a pair of transistors, said differentialamplifier circuit receiving said charging voltage as a control signalfor controlling one transistor, and a predetermined reference voltage asa control signal for controlling another transistor.
 7. The electronicapparatus according to claim 6, wherein said differential amplifiercircuit has a third constant current source connected to one end of saidone transistor and to one end of said another transistor of saiddifferential amplifier circuit, the other end of said one transistor andthe other end of said another transistor are connected to respectiveload means, said one transistor outputting from said other end thereofsaid first direction voltage, and said other transistor outputting fromsaid other end thereof said second direction voltage.
 8. The electronicapparatus according to claim 6, wherein said predetermined referencevoltage is set between said first and said second charging voltages. 9.The electronic apparatus according to claim 1, wherein each of saidmultiplicity of current control circuits includes: a first seriescircuitry consisting of a first direction selection switch adapted to beturned on by said first direction current selection signal and a firstdirection current control element for flowing a first direction currentin accord with said first direction voltage, and a second seriescircuitry consisting of a second direction selection switch adapted tobe turned on by said second direction current selection signal and asecond direction current control element for flowing a second directioncurrent in accord with said second direction voltage, said first andsecond series circuitry connected in parallel to each other.
 10. Anelectronic apparatus having audio output units, comprising: amultiplicity of output amplifier circuits each having a differentialamplifier having a first input terminal for receiving an audio inputsignal, a second input terminal for receiving a reference voltage, andan output terminal for outputting the output signal of said amplifiercircuit, and a feedback resistor connected between said first inputterminal and said output terminal, said first input terminal capable ofreceiving a supplied DC current; a common time-constant circuit having acondenser and a charge and discharge circuit connected to saidcondenser, said time-constant circuit receiving a direction instructionsignal instructing charging or discharging of said condenser, andcharging said condenser to a first predetermined voltage with apredetermined time constant upon receipt of a direction instructionsignal instructing charging of said condenser, but discharging saidcondenser to a second charging voltage with said predetermined timeconstant upon receipt of a direction instruction signal instructingdischarging of said condenser, said time-constant circuit therebyoutputting the charging voltage of said condenser; a common currentinstruction circuit receiving said charging voltage, said common currentinstruction circuit outputting a first direction voltage that increases(decreases) as said charging voltage increases (decreases) and a seconddirection voltage that decreases (increases) as said charging voltageincreases (decreases); and a multiplicity of current control circuits,each current control circuit connected to a corresponding one of saidoutput amplifier circuits and each receiving said first directionvoltage, second direction voltage, a first direction current selectionsignal, and a second direction current selection signal, and selectivelyoutputting, to said corresponding output amplifier circuit, either oneof a first direction current in accord with said first directionvoltage, a second direction current in accord with said seconddirectional voltage, a sum of said first and second direction current,or zero current according to the combination of said first directioncurrent selection signal and second direction current selection signal,wherein when any one of said multiplicity of output amplifier circuitsin operation is shut down, the charging voltage of said time-constantcircuit is reversed and the DC current supplied from the correspondingcurrent control circuit is increased; when any one of said multiplicityof output amplifier circuits not in operation is started up, thecharging voltage of said time-constant circuit is reversed and the DCcurrent supplied from the corresponding current control circuit isdecreased; and when the state(s) of only one or some of saidmultiplicity of output amplifier circuits are to be changed individuallyfrom non-operating state to operating state or vise versa, DC currentssupplied from those current control circuits associated with theremaining output amplifier circuits to said remaining output amplifiercircuits are controlled not to change through said change of states,irrespective of any change in the charging voltage of said condenser ofsaid time-constant circuit.